Characterization
and Stability of Acid Mine Drainage Tratment Sludge
Mine Environment Neutral Drainage at CANMET-MMSL |
MEND Report
3.42.2a
May 1997
SUMMARY
Acid mine drainage
(AMD) and other acidic metalliferous effluents are commonly treated
by the mining and metallurgical industries by lime neutralization.
Upon neutralization, metals precipitate out of the effluent as hydroxides.
This neutralization produces voluminous hydroxide sludges with low
solids content (frequently < 5%). Despite recent improvements
to the traditional neutralization method, it is estimated that as
much as 6.7 million cubic metres of sludge are produced annually
in Canada. In addition, the Canadian mineral industry is faced with
questions related to the long term stability of AMD treatment sludges,
and their environmentally acceptable disposal.
There is a
need to develop standard sampling, handling and characterization
protocols for AMD treatment sludges. A systematic method of assessing
sludges with respect to their chemical, physical and leaching characteristics
is necessary for estimating the sludge stability and for making
informed decisions for disposal.
This report
summarizes work that has been carried out in three areas:
- a site survey
and sampling campaign of AMD treatment sludges at 11 Canadian
mine sites;
- a detailed
characterization of the collected sludges including physical,
chemical, mineralogical and thermal analyses; and
- the leaching
of the sludge samples using two distinct tests in association
with a review of current hazardous waste regulations.
This report
provides a data bank of lime treatment sludge characteristics which
has been applied here in the discussion of sludge stability. Furthermore,
this information may be used to assist operators, researchers and
regulators in the development of improved treatment processes, effective
disposal methods and appropriate regulatory tests for lime treatment
sludges. These data may also be used to compare treatment operations
and to forecast sludge related issues arising at treatment plants.
Sludge sampling
Sludge samples
were collected from 11 Canadian mine sites (seven base metal, two
uranium, one gold and one coal) from December 1995 to March 1996.
Background information on AMD production, sludge production and
disposal as well as the overall treatment process was compiled for
all sites. Wherever possible, both fresh (i.e., end of pipe) and
aged (i.e., pond core samples at depth) sludge samples were collected
to study the effects of natural sludge aging.
Sampling plans
were developed prior to the collection of sludge cores to ensure
a representative composite sample of aged sludge. The number of
samples was based in large part on the volume of disposed sludge,
while the sampling stations were defined after a review of site-specific
characteristics including pond dimensions and patterns of disposal
within the pond. A commercially available hand corer with extension
was used to reach depths of up to 6 m and is recommended for collection
of cores in shallow water and/or sampling through an ice cover in
winter.
Sludge characterization
Physical characterization
and leaching tests were performed on the wet samples. The remaining
analyses (chemical, mineralogical and thermal) were done using the
freeze-dried material.
The pH values
for the sampled sludges were alkaline and ranged from 8.2 to 10.8.
In most cases aged sludges showed a lower pH than their fresh counterparts.
Eh values ranged from 58 to 315 mV with the aged sludges
commonly recording the lower values.
Denser sludges,
generally produced using the High Density Sludge (HDS) process,
displayed both smaller median particle sizes and narrower particle
size distributions. Many of the sludges produced from conventional
or basic treatment processes exhibited bimodal particle size distributions.
In all but one case, the measured particle size was greater for
the aged sludge.
The solids
content of the sludges ranged from 2.4% to 32.8%. In almost all
cases at least a 25% increase in solids content was seen from the
fresh to the aged material. Based on the samples tested, no correlation
was observed between the degree of densification and either the
age of the deposited sludge or the presence (or absence) of a water
cover on the sludge pond.
Neutralization
potential values for the sludges collected ranged from 108 to 725
tonnes CaCO3 equivalent per 1000 tonnes sludge. While
low NP values are attractive in terms of plant efficiency, sludges
with high NPs have more neutralization capacity which directly impacts
on long term sludge stability. Calcium content in the sludges varied
from 3.8% to 27%; calcium is present in two main forms, as calcite
or gypsum. The metals content of sludge can be viewed as potential
recoverable assets or a source of leachable metals. Zinc
recovery may be possible for some sludges ([Zn]>14%). Zinc concentrations
ranged from 0.019% to over 14.4%. The low concentrations observed
for copper and nickel (generally less than 1%) do not justify their
recovery. Aluminum ranged from 0.1% to 11%. Copper, arsenic, boron,
cadmium, chromium, mercury, lead, and selenium occur only in trace
amounts, generally less than 0.01%. Iron ranged from 1.5% to 28%
in the sludges.
All the sludges
contained sulphate, in some cases greater than 30%. The sulphate
content correlated directly with the amount of total sulphur in
most of the sludge samples, indicating that all the sulphur present
in these samples occurs as sulphate.
Mineralogical
analyses of all sludge samples showed a major amorphous phase. Readily
leached metal species such as zinc were commonly associated with
this phase, which appeared to be effective in scavenging metal species
(Al, Cu, Fe, Mg, Na, Ni, Zn) during precipitation. Calcium is present
as calcite, gypsum and bassanite; they occur both as individual
grains and in the amorphous phase. The amount of calcite may indicate
the degree of recrystallization and the increased stability of the
sludges. Quartz, silicates, sulphides and iron oxide particles found
in the sludges are detrital in origin.
Sludge leachability
AMD treatment
sludge samples were leached using two protocols. The Ontario Leachate
Extraction Procedure (LEP) uses an acetic acid solution as a leachant
while the Modified LEP substitutes a synthetic acid rain for the
acetic acid. Acetic acid mimics the organic acids expected to be
present in a municipal landfill and assumes co-disposal of mineral
processing and municipal wastes. On the other hand, the mixture
of sulphuric and nitric acids better simulates the inorganic acids
that are likely to come in contact, through acidic precipitation,
with sludges disposed in ponds. Generally, less metal was leached
from the sludges when they were subjected to the Modified LEP as
opposed to the Ontario LEP. Sludge leachability is strongly dependent
upon the final leachant pH which is influenced by the choice of
leachant and by the neutralizing potential of the sludge. Metal
leachability increases with decreasing pH at pH less than about
9.5. The amount of metal leached is also related to the metal concentration
in the sludge itself. In general, the aged sludge samples showed
an increase in stability relative to the fresh samples as indicated
by the leaching results and supported by the mineralogical data
and particle size analyses.
AMD treatment
sludges are waste products which may be subject to waste management
regulations. A leachate extraction test may be used to evaluate
if the waste is capable of yielding a leachate which exceeds regulated
concentration limits for selected contaminants. When a waste fails
the test in relation to the limits specified in a particular jurisdiction,
the waste may be classified as hazardous. All but two of the sludge
samples subjected to the Ontario LEP passed the test when the leachate
concentrations for metals are compared to the regulated limits governing
the classification of hazardous waste material in Canadian jurisdictions.
A fresh sludge from a base metal operation failed on zinc and an
aged sludge from a uranium mine failed on uranium. There are only
three jurisdictions in Canada which have a regulated limit for zinc
and this sludge would actually fail only in comparison to Québecs
current regulation. It must be noted however, that Québecs
proposed new regulated limits do not include zinc. None of the sludge
samples failed when tested with the Modified LEP. The leachate concentrations
from both tests were generally at least five times lower than the
most stringent of the regulated limits.
Therefore,
fresh AMD treatment sludges would not generally be classified as
hazardous wastes based on current leaching protocols and regulated
contaminant limits. Aged sludges are even less likely to be classified
as hazardous wastes. Based on the samples tested, this work has
underlined that while sludge stability is an issue, greater emphasis
should be placed on sludge disposal and volume reduction.
Numerous leach
protocols have been developed to test solid wastes. None of these
leaching tests have been specifically designed for evaluating AMD
treatment sludge leachability. A thorough review of regulatory and
research leach protocols from Canada and the United States is provided.
While regulatory leach tests for classification of hazardous wastes
in Canada involve the use of acetic acid in the protocol, more appropriate
tests, such as the Modified LEP, need to be considered for assessing
the leachability of AMD treatment sludges for on-site disposal in
a pond environment. Ultimately, the context within which sludge
leachability (stability) is measured must be kept in mind when a
leach test is applied.
In most provinces
and territories, the testing of AMD treatment sludge and its storage/disposal
is controlled by site-specific licences or permits based on appropriate
legislation. A review of Canadian and American hazardous waste regulations,
and other pertinent regulations and guidelines as they apply to
the leachability testing of AMD treatment sludges is provided.
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